Ophiogaleus, , Thuy, 2013

Rousseau, Julie, Gale, Andrew Scott & Thuy, Ben, 2018, New articulated asteroids (Echinodermata, Asteroidea) and ophiuroids (Echinodermata, Ophiuroidea) from the Late Jurassic (Volgian / Tithonian) of central Spitsbergen, European Journal of Taxonomy 411, pp. 1-26 : 12-16

Ophiogaleus sp.

Figs 4–5

Material examined

NORWAY: central Spitsbergen, Janusfjellet , 78°20′35.4″ N, 15°49′85.2″ E, Lower Middle Volgian ( Middle Tithonian ), Slottsmøya Member , Agardhfjellet Formation ( PMO 217.899a ); central Spitsbergen, Konusdalen , 78°19′97.1″ N, 15°52′15.5″ E, from the same layer of sideritic concretions ( PMO 218.053a , PMO 218.060 ). In total, 9 specimens were collected from the Janusfjellet and Konusdalen sites.

Description

PMO 217.899a ( Fig. 4 ) is an articulated disc with five fragmentary arms and with the splitting plane close to the ventral side of the skeleton. The disc is distorted and the reconstructed diameter is 14.9 mm (average of narrowest 11.7 mm and broadest 18.0 mm).

The oral plates are long and devoid of lateral wings. The dental plates ( Fig. 4 B–C) have two or three very small, blunt oral apical papillae. The lateral oral papillae are not unambiguously discernible, but seem to be similar to the apical ones. The adoral shields are long and slender, but their exact outline is not visible. The oral shields ( Fig. 4 B–C) are small and nearly pentagonal with concave proximal and latero-distal edges.

The interradial areas ( Fig. 4 B–C) are not indented and covered by moderately dense, small, conical spines, three to four times as long as thick. The interradial disc scales, if present at all, must have been very thin. The radial shields are elongated, oval to slightly pear-shaped, around three times as long as broad and about one fourth of the disc radius. The adradial genital plates are long and slender, longer

than the radial shield, while the abradial genital plates are bar-like and a third of the adradial genital plates in length.

The six basalmost arm segments are incorporated into the disc. The longest preserved arm has a length of 24.5 mm from the edge of the disc onwards and comprises 20 segments. The arms taper very slowly, suggesting a considerable original arm length. The proximal arm segments are wider than long, while the distalmost arm segment is nearly as wide as long. The tentacle pores are moderately large, developed as between-plate openings on all segments preserved. The lateral arm plates present a strongly bulging distal portion bearing spine articulations, which gives a clearly noded aspect to the arm segments ( Fig. 4D ). The outer surface of the lateral arm plates seems to be coarsely reticulated, but details are not well preserved. The spine articulations are ear-shaped, large and free-standing on the bulging distal portion of the lateral arm plates. The spines are erect, slender, conical and seemingly smooth. The spine length equals the length of two segments. There are at least five spines proximally. The lateral arm plates abut ventrally from the proximal-most segments onwards. The outline of the ventral arm plates is not discernible on specimen PMO 217.899a.

PMO 218.060 ( Fig. 5 A–E) is an articulated disc preserving portions of all five arms and visible on two slabs of rock as a part (PMO 218.060a, Fig. 5A ) and counterpart (PMO 218.060b, Fig. 5B ). The specimen is similar to PMO 217.899a with respect to disc and arm morphology, but supplements the latter in some aspects. The lateral oral papillae are small, elongate, oval, and similar in size and shape to the apical oral papillae. Due to incipient disarticulation, the disposition of the lateral oral papillae is not clearly visible, although it seems as if they formed a single loosely contiguous row along the jaw edge with at least five lateral papillae per half-jaw. The interradial disc scales are better visible than in the previously described specimen. They are thin, minute, and circular to oval in outline ( Fig. 5A, C ). One median to distal arm segment ( Fig. 5E ) is preserved in cross section and shows a dorso-ventrally compressed, oval arm section with the ventro-distal portions of the lateral arm plates slightly protruding ventralwards and at least seven arm spines per lateral arm plate forming an almost continuous collar around the arm.

PMO 218.053a ( Fig. 5F ) is a partly disarticulated arm portion composed of three median segments. The specimen is well in agreement with those previously described in terms of arm morphology, but provides further insights into the ventral side of the arms. The ventral portion of the lateral arm plate is wide and contiguous over half of the arm segment length. The ventral arm plate is roughly diamondshaped, much wider distally than proximally, with convex distal edge, concave latero-proximal edges and pointed proximal tip. The tentacle pores are encompassed by lateral and ventral arm plates and covered by two elongate, rounded triangular tentacle scales.

Remarks

The long, erect arm spines positioned on the strongly bulging distal portion of the lateral arm plates, giving the arms a conspicuously noded aspect, in combination with the disc spines, the ear-shaped arm spine articulations and the oral plates devoid of lateral wings, unquestionably place the Spitsbergen specimens in the suborder Ophiacanthina (formerly the family Ophiacanthidae as understood prior to the new classification of O’Hara et al. 2017). Within this group, closest affinities are shared with Ophiogaleus , an extinct genus described by Thuy (2013) on the basis of articulated arm fragments and dissociated lateral arm plates. Shared features pertain to the lateral arm plates with the strongly bulging and ventrally protruding distal portion, the wide ventral portion in median to distal arm segments, and the coarsely meshed outer surface stereom, the numerous arm spines forming a nearly continuous fan dorsally, as well as the rounded triangular tentacle scales. No other known extant or extinct ophiacanthinid presents this combination of characters. We thus assign the here-described specimens to Ophiogaleus , albeit in open nomenclature as the lateral arm plates preserve insufficient detail for an unambiguous species-level identification.

The present material falls within the known stratigraphic range of Ophiogaleus (Sinemurian to Maastrichtian) ( Thuy 2013), but extends the palaeogeographical range of the genus to polar latitudes. It furthermore stands out in representing the first finds of complete, articulated skeletons of Ophiogaleus providing insights into the previously unknown disc morphology and allowing to significantly extend the diagnosis of the genus. Since exhaustively known fossil ophiuroids play a pivotal role in phylogenetic estimates ( Thuy & Stöhr 2016), the Spitsbergen Ophiogaleus material significantly adds to the dataset available for the systematic reconstruction of ophiuroid evolutionary history ( O’Hara et al. 2017).

In the original description of Ophiogaleus, Thuy (2013) postulated close phylogenetic ties with the extant genus Ophiacantha Müller & Troschel, 1842 on the basis of similarities in lateral arm plate morphology. Indeed, the new insights on disc morphology of Ophiogaleus fall within the spectrum of the extant members of the family Ophiacanthidae as recently circumscribed by O’Hara et al. (2017) with its type genus Ophiacantha . However, due to the absence of family-level diagnoses for the new classification by O’Hara et al. (2017), it is currently not possible to provide compelling evidence in favour of such an assignment. Ophiogaleus might, in fact, belong to a yet unknown extinct family sister to the Ophiacanthidae . With the present knowledge at hand, however, the most conservative approach is an assignment to the Ophiacanthidae .

Ophiogaleus sp. was probably a suspension feeder and used its long, robust, erect arm spines to trap food particles, as do some modern suspension-feeding ophiacanthids ( O’Hara & Stöhr 2006; Hess & Meyer 2008). Its occurrence at shelf depths is not unusual as the late Jurassic was a time of exceptionally abundant and diverse ophiacanthid records in shallow waters ( Thuy 2013). Modern ophiacanthids are most common and diverse at bathyal depths, although shallower occurrences are not uncommon, especially at high latitudes.

Figures

Fig. 4. Ophiogaleus sp. PMO 217.899a. A. Specimen preserving the ventral side of the disc area, two nearly complete arms and proximal area of a third arm. B. Detail view of the disc area. C. Drawing detail of the area shown in B. D. Detail view of a proximal section of arm. Abbreviations: abgp = abradial genital plate; adgp = adradial genital plate; ds = disc spine; lopa = lateral oral papillae; op = oral plate; sart = spine articulation; v = vertebra.

Fig. 5. Ophiogaleus sp. A–E. PMO 218.060, specimen preserving the ventral side of the disc area and the base of all five arms. The specimen has been split through a horizontal plane and is visible as part (A) and counterpart (B). Photographs in A and B by H.A. Nakrem. C. Drawing detail of the oral and interradial area. D. Detail view of proximal arm segments with articulated spines. E. Detail view of distal arm segment. F. PMO 218.053a, partly disarticulated arm portion composed of three median segments showing details of the ventral side of the arms. Abbreviations: abgp = abradial genital plate; adgp = adradial genital plate; as = arm spine; cs = arm in cross section; dp = dental plate; ds = disc spine; lap = lateral arm plate; op = oral plate; rs = radial shield; vap = ventral arm plate.